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Structure of the allyl group.

An allyl group is a hydrocarbon group with the structural
formula H2C=CH-CH2. It is made up of a vinyl group, CH2=CH,
attached to methylene
CH2. The name derived from the Latin word for garlic, Allium sativum.
Theodor
Wertheim isolated an allyl derivative from garlic oil and named
it Schwefelallyl.[1]

Contents

Nomenclature and bonding

Allyl is a widely used term in organic chemistry.[2] Allyl
itself is a relatively stable free radical with the
formula C3H5. It contains three
sp²-hybridized carbon centers. The unpaired electron is
delocalized. Allylic anions and cations are often discussed as
intermediates in reactions.

MO diagram for pi-orbitals on allyl. The middle MO labeled
Ψ2 is singly occupied in the allyl radical. In the allyl
cation Ψ2 is unoccupied and in the allyl anion it is
doubly occupied. Hydrogen atoms are omitted from this
picture.

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Allylic
sites

A site on the saturated carbon atom is called the "allylic
position" or "allylic site." A group attached at this site is
sometimes described as "allylic." Thus,
CH2=CH2CH2OH "has an allylic hydroxyl group." Allylic C-H bonds are
about 20% weaker than the C-H bonds in ordinary sp3
carbon centers and are thus more reactive. As one practical
consequence of this heightened reactivity, the production of acrylonitrile
exploits the easy oxidation of the allylic C-H centers in propene. Benzylic and allylic are
related in terms of structure, bond strength, and reactivity. Other
reactions that tend to occur with allylic compounds are allylic oxidations, ene reactions, and the Trost asymmetric
allylic alkylation.

Homoallylic

The term homoallylic refers to the position on a carbon skeleton
next to an allylic position. In
CH2=CHCH2CH2Cl, the chloride
occupies a homoallylic position.

Allyl
compounds

Many substituents can be attached to the allyl group to give
stable compounds. Allyl alcohol has the structure
H2C=CH-CH2OH. It contains two
sp²-hybridized carbon centers and one sp3-hybridized
carbon center. Another example of a simple allyl compound is allyl chloride.
Substituted versions of the parent allyl group, such as the
trans-but-2-en-1-yl or crotyl group
(CH3CH=CH-CH2-), may also be referred to as
allylic groups. Many allylic compounds are lacrymatory.

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Allyl ligands in
organometallic chemistry

The allyl ligand is commonly encountered in organometallic chemistry. Most
commonly, allyl ligands bind to metals via all three carbon
centers, the so-called η3-binding mode.
η1-allyl complexes are also known. The most popular
allyl reagent is probably allyl
palladium chloride. They are usually generated by oxidative
addition of allylic halides. This route affords pi-allyl nickel
compounds, such as
(allyl)2Ni2Cl2:[3]